Piston Design for Flow Re-Direction

ABSTRACT

A piston for an internal combustion engine includes a crown portion having a bowl that includes a plurality of protrusions. Each of the plurality of protrusions includes a first side surface and a second side surface. Other features including at least one ledge formed between protrusions in segments, and a generally flat, inward facing surface on the protrusions may also be used.

TECHNICAL FIELD

This patent disclosure relates generally to internal combustion enginesand, more particularly, to combustion chamber features fordirect-injection engines.

BACKGROUND

Most modern engines are direct-injection engines, which means that eachcombustion cylinder of the engine includes a dedicated fuel injectorconfigured to inject fuel directly into a combustion chamber. Whiledirect-injection engines represent an improvement in engine technologyover past designs, in the form of increased engine efficiency andreduced emissions, the improvement of the design of any particularengine is always desirable, especially in light of increasing fuel costsand ever more strict regulations on engine emissions.

In a traditional direct-injection engine, one or more fuel jets that areinjected into a combustion chamber interact with various combustionchamber structures, which cause the fuel to disperse into the combustionchamber. More specifically, the fuel jet(s) entering the combustionchamber impact various surfaces of the combustion chamber such as apiston bowl, the flame deck surface of the cylinder head, the cylinderliner or bore, and other surfaces before spreading in all directions.The impingement of the fuel jets with these structures may have avariety of effects including increased emissions because localized areashaving higher fuel concentrations may burn rich, while other areas inthe combustion chamber may burn lean. Following interaction with thevarious internal surfaces of the combustion chamber, the fuel jets andresulting flames may also interact with neighboring fuel jets or flames.These interactions can further result in higher temperatures, decreasedfuel efficiency, increased heat rejection and component temperatures,and the like.

Various solutions have been proposed in the past for improving anengine's efficiency and reducing its emissions. One example of apreviously proposed solution can be seen in U.S. Pat. No. 8,646,428(“Eismark”), which was granted on Feb. 11, 2014. Eismark describes apiston having a crown in which protrusions having a smooth form areadapted for preserving kinetic energy in a flame plume. The piston isdesigned to be used in an engine in which quiescent air is provided inthe engine cylinder. The fuel injector provides fuel jets or flames intothe cylinder that impinge on features formed in the piston bowl toredirect portions of the flames upward, towards a cylinder head surface,and the remaining portions of the flames in a tangential direction,within the bowl, to achieve better mixing of the combustion gases anddecrease or eliminate stagnation zones in a combustion chamber.

While the flow redirection of Eismark may be partially effective inimproving burning of fuel in an engine cylinder, it is configured tooperate with a quiescent cylinder, which is difficult to attain for eachcylinder consistently. In a typical engine, the momentum of intake airinto an engine cylinder will possess at least some swirl, whichfollowing fuel injection into the cylinder will cause the flames thatdevelop to be carried by the swirling air to one side and generallytowards the cylinder wall.

SUMMARY

The disclosure describes, in one aspect, an internal combustion engine.The internal combustion engine includes an engine block having acylinder bore, a cylinder head having a flame deck surface disposed atone end of the cylinder bore, and an air intake valve associated withthe cylinder head and configured to open and allow a flow of intakecharge into the cylinder bore. A piston is connected to a rotatablecrankshaft and configured to reciprocate within the cylinder bore. Thepiston has a crown portion facing the flame deck surface such that acombustion chamber is defined within the cylinder bore and between a topsurface of the crown portion and the flame deck surface. The crownportion includes a piston bowl having a generally concave shape andextending within the crown portion and a wall, the wall extendingperipherally around the piston. A fuel injector has a nozzle tipdisposed in fluid communication with the combustion chamber. The nozzletip has a plurality of nozzle openings configured to inject a pluralityof fuel jets into the combustion chamber, each of the plurality of fueljets being provided along a respective fuel jet centerline.

In one embodiment, a plurality of protrusions is disposed in the pistonbowl adjacent the wall. Each of the plurality of protrusions includes afirst side surface and a second side surface. At least one ledge extendsat least partially through an inner face and towards an outer face ofthe wall. The at least one ledge extends at a depth that is less than adepth of the piston bowl.

In another aspect, the disclosure describes a piston for an internalcombustion engine. The piston includes a piston body, a crown portionextending below a top surface of the piston body, the crown portionincluding a bowl having a generally concave shape and extending withinthe crown portion and a wall, the wall extending peripherally around thepiston body, a plurality of protrusions disposed in the piston bowladjacent the wall, each of the plurality of protrusions including afirst side surface and a second side surface, and at least one ledgeextending at least partially through an inner face and towards an outerface of the wall, the at least one ledge extending at a depth that isless than a depth of the piston bowl.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of an engine combustion chamber in accordancewith the disclosure.

FIG. 2 is a top view in cross section of an engine piston in accordancewith a first embodiment of the disclosure.

FIG. 3 is an enlarged, partial section through a sidewall of the enginepiston of FIG. 2.

FIG. 4 is a second alternative embodiment for a protrusion in accordancewith the disclosure.

FIG. 5 is a third alternative embodiment for a protrusion in accordancewith the disclosure.

FIG. 6 is a fourth alternative embodiment for a protrusion in accordancewith the disclosure.

DETAILED DESCRIPTION

This disclosure relates to internal combustion engines and, moreparticularly, to features incorporated within at least one combustionchamber of the engine to redirect fuel jets provided by separate fuelinjector nozzle openings towards a center portion of the combustionchamber, for example, towards a fuel injector, even for cylinders inwhich incoming air or an intake charge, which intake charge may includea mixture of air, fuel and/or recirculated exhaust gas, may includeswirl. In the present disclosure, the term “jets” or “fuel jets”describes reacting (i.e. burning) or non-reacting streams of fuel, aloneor in mixture with air, that is provided into an engine cylinder. Thesefuel jets may therefore comprise fuel droplets dispersed in air, or aflame once the fuel begins to oxidize with the surrounding air. Inaccordance with the disclosure, the fuel jets are redirected and alsosegregated during a majority of the injection time and/or burn time topromote better oxygen utilization within the combustion chamber ascompared to previously proposed or known combustion systems.

The various exemplary embodiments described herein include structuresand features that operate or result in redirecting fuel jets radiallywith respect to the cylinder bore of an engine, to thus minimize or, atleast, delay interaction between adjacent fuel jets entering thecombustion chamber. The type of fuel being provided to the cylinder maybe a spray of liquid fuel such as diesel or gasoline, or a jet ofgaseous fuel such as natural or petroleum gas. The design is configuredto impart an asymmetric effect to a combined air and fuel moving mass offluids, which initially have a swirling aggregate velocity vector andwhich are redirected to have an aggregate radial velocity vector towardsa center of the piston bore.

Stated differently, air or an intake charge, which may include air inmixture with recirculated exhaust gas and/or fuel, entering into thecylinder may have swirl, i.e., radial and primarily tangential velocitycomponents of each air particle. As fuel is injected into the chamber,it mixes with the air. The fuel particles or droplets have generally aradial velocity component such that, when the fuel droplets mix andevaporate into the swirling air, the tangential velocity component ofthe resulting mixtures is reduced, but not eliminated. The remainingtangential velocity component is countered by uneven or asymmetricalsurfaces presented on protrusions on the piston, which impart acounter-swirl tangential velocity component to the fuel/air mixture thatimpinges on and is affected by the protrusion surfaces. Thecounter-swirl tangential velocity component of the fuel/air mixture thuscancels or eliminates the original tangential velocity of the swirlingair mass, such that the resulting fuel/air mixture possesses a radiallyinward velocity component. In this way, a burning air/fuel mixture isdirected inwardly relative to the piston, where additional oxygen tosupport the burning fuel is available. The disclosed embodiments can betailored to counter many different particular swirling patterns that mayexist in engine cylinders, and essentially transform a swirlingcombustion system into a quiescent combustion system. Some of thebenefits of such a combustion system include reduced heat rejection, inthat the flame is guided towards the center of the cylinder and awayfrom the metal structures of the engine that surround and define thecylinder, which in turn leads to lower component temperatures, increasedfuel efficiency, and a more uniform fuel/air mixture, which also leadsto lower engine emissions.

A cross section of a combustion chamber 100 of an engine 101 inaccordance with the disclosure is shown in FIG. 1. The combustionchamber 100 has a generally cylindrical shape that is defined within acylinder bore 102 formed within a crankcase or engine block 104 of theengine. The combustion chamber 100 is further defined at one end by aflame deck surface 106 of a cylinder head 108, and at another end by apiston crown 110 of a piston 112 that is reciprocally disposed withinthe cylinder bore 102. A fuel injector 114 is mounted in the cylinderhead 108. The fuel injector 114 has a tip 116 that protrudes within thecombustion chamber 100 through the flame deck surface 106 such that itcan directly inject fuel into the combustion chamber 100.

During operation of the engine 101, air or an intake charge is admittedinto the combustion chamber 100 via an air inlet passage 115 when one ormore intake valves 117 (one shown) are open during an engine stroke, forexample, at least a portion of an intake stroke, compression strokeand/or exhaust stroke. As is the case in most engines, an incomingairflow into the combustion chamber 100 through the one or more intakevalves 117 will be highly turbulent and possess swirling portions aroundone or more axes, which are imparted into the air flow by the variousbends and corners in the air inlet passage 115 and other structures suchas air passing over and around the intake valve 117. In a knownconfiguration, high pressure fuel is permitted to flow through aplurality of nozzle openings in the tip 116. Each nozzle opening createsa fuel jet 118 that generally disperses to create a fuel/air mixture,which in a compression ignition engine auto-ignites and combusts. Thefuel jets 118 may be provided from the injector at an included angle, β,of between 110 and 150 degrees, but other angles may also be used. Thefuel jets 118 enter the combustion chamber 100 in a generally radiallyoutward direction as the fuel travels through the injector openings.Following combustion, exhaust gas is expelled from the combustionchamber through an exhaust conduit 120 when one or more exhaust valves122 (one shown) is/are open during an exhaust stroke and/or intakestroke.

The uniformity and extent of fuel/air mixing in the combustion cylinderis relevant to the combustion efficiency as well as to the amount andtype of combustion byproducts that are formed and to the rate ofcombustion within the combustion chamber. For example, fuel-richmixtures, which may be locally present within the combustion chamber 100during a combustion event due to insufficient mixing, or insufficientair available locally around those areas, may lead to higher soot,hydrocarbon, and carbon monoxide emissions and lower combustionefficiency. In the illustrated embodiments, improved fuel/air combustionis managed for each fuel jet by forming a plurality of protrusions,which asymmetrically funnel or guide flames created when streams of airand fuel in the cylinder burn. This flame guiding is also helpful inachieving a more complete combustion within the cylinder, which canlower soot and other emissions of the engine. The direction of flamepropagation after the flames have been redirected by interaction withfeatures in the piston bowl is such that at least a portion of theflames is directed to counter a swirl present in the cylinder. The neteffect of the directed flames and swirling air is a direction of theflames towards the center of the piston, where air is available to burnan air/fuel mixture and oxidize soot. Each protrusion has two sides, afirst side that has a generally concave shape and a normal vector thatfaces towards or against a swirling direction, and a second side thathas a generally flat and/or convex shape and a normal vector that faceswith or in the same direction as the swirling direction of air in thecombustion chamber 100. In this way, either side of each protrusionserves to accept, redirect and segregate a portion of each of twoadjacent fuel jets originating from the plurality of nozzle openings inthe fuel injector, and redirect them towards the center of thecombustion chamber 100.

A first exemplary embodiment of the piston 112 is shown in FIG. 2. Inthe illustration of FIG. 2, only a top surface 200 of the piston crown110 of the piston 112 is shown in cross section from a top perspectivefor illustration. The piston 112 includes a bowl 124 formed in thepiston crown 110 that includes a central, raised conical protrusion 126at the center of a conical, convex surface 128. The bowl 124 has agenerally circular periphery and is defined within a circular crown wall130. Included in the top surface 200 is a plurality of protrusions 202,which are disposed within the bowl 124 and along a periphery of the bowl124 adjacent the wall 130. Six protrusions 202 are shown herein, but itshould be appreciated that any number of protrusions can be useddepending on the number of nozzle openings in the tip 116 of theinjector. The protrusions in the top surface 200 are arranged at regularintervals along equally distributed radial axes 204. Each axis 204 isdisposed at exactly the same angle between the spray directions ofadjacent nozzle tip openings of the fuel injector 114 such that a fueljet 118 will emanate from the tip 116 in a radial direction between twoadjacent axes 204, as shown.

In the schematic embodiment shown in FIG. 2, the development of twoadjacent fuel jets 118 is shown at different instances in time. Thelower, not fully developed jet to the right of the figure is shown at aninstant when the fuel jet 118 has been emanated from the tip 116 but hasnot yet reached the wall 130. During this, initial time in an injection,the air in the cylinder and, thus, in and above the bowl 124, may have agenerally circular or spiral momentum in a counterclockwise directionindicated by the block arrows “S.” While the fuel jet 118 is travellingthrough a moving region of air, the speed of the air, which may includea tangential velocity vector because of the swirling momentum may notaffect the radial travel direction of the fuel jet 118, at leastinitially. However, at a later instant, as shown by the fuel jet 118shown counterclockwise and to the left of the original jet, towards thetop of the figure, the fuel jet or flame 118 may impinge against thewall 130 and separate into two tangential jets, each tangential jetheading towards the two adjacent protrusions 202 that flank theimpingement site.

As can be seen in FIG. 2, a segment of the wall 130 between adjacentprotrusions includes a recessed or stepped-lip feature or ledge 132extending at least partially through an inner face 134 of the wall 130towards an outer face 136 of the wall 130. At least a portion of theledge 132 from a side perspective is shown in the enlarged detailedcross section of FIG. 3. The ledge 132 in the illustrated embodimentdoes not extend peripherally along the entire inner circumference of thewall 130, but is rather formed in segments, each segment spanning achordal distance along the inner circumference of the wall 130 betweenadjacent protrusions 202. Each ledge 132 is curved to follow the contourof the wall 130 and has a peripheral width along a radial direction thatis less than a thickness of the wall 130 in that direction. Each ledgefurther has a depth, D, in the axial direction that corresponds to adistance traveled by the piston in the bore for a given rotation ofengine's crankshaft. An outer edge 138 of the ledge 132 can be embodiedas a sharp corner, as shown in FIG. 3, or as a blended, chamferedsurface smoothly connecting a ledge floor 140 with an outer ledge wall142.

During engine operation, the timing for providing one or more injectionevents for the fuel jets 118 may be selected such that the fuel jets 118at least partially impinge into the ledges 132 as the piston 112 ismoving upward into the bore during a compression stroke and/or downwardduring an expansion stroke. The recesses provided by the ledges 132 mayallow at least some fuel to proceed radially outwardly relative to thepiston and enter the so-called “squish region,” which describes an upperand radially outward hollow cylindrical volume within the piston borethat is disposed between the flame deck surface and a top, annularsurface of the wall 130 and also, in this embodiment, an aggregatevolume of the ledges 132. As the piston progresses upward, the same orsubsequent fuel jets will no longer overlap the ledges and impinge loweron the inner surface 134 of the wall 130 to be redirected towards aninner portion of the combustion chamber. In another embodiment, delayedtiming of the fuel jets 118 and the flames created thereby during anexpansion stroke, i.e., when the piston is moving downward in thecylinder, the fuel jets 118 may be directed inward relative to thecombustion cylinder and then, at least momentarily, overlap the ledges132 as the piston 112 is descending and redirect at least a portion ofthe fuel jets 118 upward towards the flame deck surface.

Turning now to the protrusions 202, these features are configured toredirect the fuel jets 118 within the cylinder towards the center of thecombustion chamber. The protrusions 202 present different profiles orshapes to asymmetrically guide the tangential jets that impinge thereonand to delay jet-to-jet interaction for improved mixing and oxidationwithin the combustion chamber. For illustration, each protrusion 202includes a first side face 206, which faces in a direction against theswirl S, and a second side face 208, which faces in a direction with theswirl S. In the nomenclature used herein, a direction in which each sideface of the protrusion “faces” means the direction in which a normalvector that is generally perpendicular to the respective side face andpoint away from the surface in an outward direction with respect to theprotrusion is pointing. Two such vectors, V1 and V2, are shown on one ofthe protrusions 202 on the left side of the figure for illustration.

In various embodiments, the first and second side faces of theprotrusions may be different from one piston to another, but they allwill share a similar trend, which is that the first side face 206 ofeach protrusion 202, i.e., on the side that faces against the swirlingdirection S, will be generally concave such that a larger turning effectis provided to fluids impinging and being redirected by the first sideface 206 to counteract the swirl that is present in the swirlingdirection S; the same trend also includes that the second side face 208of each protrusion 202, i.e., on the side that faces away from theswirling direction S, will be generally flat or convex such that alesser turning effect is provided to fluids impinging and beingredirected by the second side face 208, because those fluids are alreadyturning in the opposite direction as the swirling direction S.

As can be seen from FIG. 2, when the fuel jet 118 impinges onto the wall130, it separates into many sub-streams that include two tangentialstreams that follow the curvature of the wall 130 until they meet theprotrusions 202 that flank the area onto which particular fuel jet 118impinges along the wall 130. These two tangential jets can be referredto as the clockwise (CW) subjet, i.e. the subjet that turns in alocation that is located in a clockwise direction relative to theparticular fuel jet 118, and the counter-clockwise (CCW) subjet, i.e.the subjet that turns in a location that is located in acounter-clockwise direction relative to the particular fuel jet 118. Fora more complete burn in the combustion chamber 100, it is desired tohave both the CW and CCW fuel subjets advance in a radial directiontowards the center of the combustion chamber. The different shapes ofthe first and second side faces 206 and 208 can accomplish this takingunder consideration the swirl S in the cylinder. Thus, the CW subjet inthe embodiment shown makes a shallow turn off the second side face 208,because it will be carried along the swirl S as it advances towards thetip 116. For the same reason, the CCW subjet makes a sharper turn offthe first side face 206, because its course will also be adjusted as itadvances against the swirl S.

An alternative or second exemplary embodiment for the protrusion 202 isshown in FIG. 4, which is a partial fragment of the piston 112 shown incross section. In this embodiment, the protrusion 202 includes agenerally flat inner face 402 that protrudes radially inwardly towardsthe center of the piston relative to remaining surfaces of theprotrusion 202. The width of the flat inner face 402 in a peripheraldirection is defined between two sharp edges 404. The sharp edges 404are disposed along an interface between the flat inner face 402 and aradially inner side of generally radially extending walls 406 disposedon either side of the protrusion 202. The radially extending walls 406meet the first and second side faces 206 and 208 of the protrusion 202as they extend in a radially outward direction. It is noted that thefirst and second side faces 206 and 208 may have different curvatures,or the same curvature. It is also noted that the walls 406 may extend atan angle relative to a corresponding radius of the piston, for example,to provide a relatively trapezoidal shape of the portion of theprotrusion defined between the flat inner face 402 and the two walls406.

A third exemplary embodiment for the protrusion 202 is shown in FIG. 5.In this embodiment, the protrusion 202 includes a generally flat innerface 502 that faces radially inwardly towards the center of the pistonrelative to remaining surfaces of the protrusion 202. The width of theflat inner face 502 in a peripheral direction is defined between twosharp edges 504. In this, and in the remaining embodiments, the term“sharp” in reference to the edges on either side of a flat face denotesurface transitions at which a flow of material impinging onto the flatface will turn and pass over the “sharp” edge while separatingtherefrom, without following the curve of the surface beyond the edge.Stated differently, the “sharp” edges have a sufficiently sharp radiusto avoid flow redirection due to Coanda effects. The sharp edges 504 aredisposed along an interface between the flat inner face 502 and thefirst and second side faces 206 and 208 of the protrusion 202.

A fourth exemplary embodiment for the protrusion 202 is shown in FIG. 6.In this embodiment, the protrusion 202 includes a generally flat innerface 602 that faces radially inwardly towards the center of the pistonrelative to remaining surfaces of the protrusion 202. The width, W, ofthe flat inner face 502 is larger than a minimum wall thickness, T, ofthe protrusion in a peripheral direction, as shown in FIG. 6. As in theembodiment of FIG. 5, the width W in the peripheral direction is definedbetween two sharp edges 604. The sharp edges 604 in this embodiment aredisposed along an interface between the flat inner face 602 and thefirst and second side faces 206 and 208 of the protrusion 202, whichfirst and second side faces 206 and 208 in this embodiment are bothconcave but at a deeper or shallower radius.

INDUSTRIAL APPLICABILITY

The present disclosure is not only applicable to internal combustionengines having reciprocating pistons, as described relative to theembodiments illustrated herein, but also to other types of applications,such as gas turbines, industrial burners and the like. In general, thevarious asymmetrical protrusions can be formed in a structure that thefuel jet will impinge upon when injected by an injector into acombustion chamber. The protrusions arcuate indents and the redirectionand segregation of fuel jets and plumes they provide are effective inpromoting faster combustion and redirection of developing flames towardsmore oxygen-rich areas at the center of the cylinder.

In addition to the desirable effects provided by redirection of flamesand/or the fuel jets that produce the flames, radially and/ortangentially relative to the piston and bore, the mixing effects thatprovide improved and more complete combustion can be improved by theledges formed between the protrusions along the inner periphery of thepiston cylindrical outer wall. As described above, the ledges may, atleast temporarily, redirect the flames axially along the reciprocatingdirection of travel of the piston, as well as radially outwardly suchthat air present in the squish region of the combustion cylinder may beutilized to oxidize fuel, soot and other compounds during combustion.

To maintain separation of the fuel jets or flames, at least initially,and to promote more vigorous mixing of the fuel with the oxidizers inthe combustion chamber, the protrusions may further include sharp edgesalong their radially inward portions, i.e., where the tangentiallyredirected flames turn to advance towards the center of the combustioncylinder. In the embodiments described above, and in other, similarembodiments, the sharp edges are disposed on either side of flat,inwardly facing faces of the protrusions. These sharp edges createedge-effects on the advancing flames to promote and enhance mixing.

It will be appreciated that the foregoing description provides examplesof the disclosed system and technique. However, it is contemplated thatother implementations of the disclosure may differ in detail from theforegoing examples. All references to the disclosure or examples thereofare intended to reference the particular example being discussed at thatpoint and are not intended to imply any limitation as to the scope ofthe disclosure more generally. All language of distinction anddisparagement with respect to certain features is intended to indicate alack of preference for those features, but not to exclude such from thescope of the disclosure entirely unless otherwise indicated.

Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein. All methods described herein can beperformed in any suitable order unless otherwise indicated herein orotherwise clearly contradicted by context.

We claim:
 1. An internal combustion engine, comprising: an engine blockhaving a cylinder bore; a cylinder head having a flame deck surfacedisposed at one end of the cylinder bore; an air intake valve associatedwith the cylinder head and configured to open and allow a flow of intakecharge into the cylinder bore; a piston connected to a rotatablecrankshaft and configured to reciprocate within the cylinder bore, thepiston having a crown portion facing the flame deck surface such that acombustion chamber is defined within the cylinder bore and between a topsurface of the crown portion and the flame deck surface, the crownportion including a piston bowl having a generally concave shape andextending within the crown portion and a wall, the wall extendingperipherally around the piston; a fuel injector having a nozzle tipdisposed in fluid communication with the combustion chamber, the nozzletip having a plurality of nozzle openings configured to inject aplurality of fuel jets into the combustion chamber, each of theplurality of fuel jets being provided along a respective fuel jetcenterline; a plurality of protrusions disposed in the piston bowladjacent the wall, each of the plurality of protrusions including afirst side surface and a second side surface; and at least one ledgeextending at least partially through an inner face and towards an outerface of the wall, the at least one ledge extending at a depth that isless than a depth of the piston bowl.
 2. The internal combustion engineof claim 1, wherein the at least one ledge is disposed between twoadjacent protrusions from the plurality of protrusions and occupies alength along an inner periphery of the wall.
 3. The internal combustionengine of claim 1, wherein the at least one ledge is formed in segments,each segment spanning a chordal distance along an inner circumference ofthe wall between adjacent protrusions.
 4. The internal combustion engineof claim 3, wherein each segment is curved to follow a contour of thewall.
 5. The internal combustion engine of claim 4, wherein each segmenthas a peripheral width along a radial direction that is less than athickness of the wall in that direction.
 6. The internal combustionengine of claim 1, wherein each of the plurality of protrusions furtherincludes a generally flat inner face that protrudes radially inwardlytowards a center of the piston bowl, the flat inner face disposedbetween the first side surface and the second side surface.
 7. Theinternal combustion engine of claim 6, wherein the generally flat innerface is defined between two sharp edges.
 8. The internal combustionengine of claim 7, wherein the two sharp edges are disposed along aninterface between the generally flat inner face and radially extendingwalls disposed on either side of the protrusion, wherein the radiallyextending walls meet the first and second side faces of a correspondingprotrusion form the plurality of protrusions.
 9. The internal combustionengine of claim 7, wherein the two sharp edges are disposed alonginterfaces between the flat inner face and the first and second sidefaces.
 10. The internal combustion engine of claim 6, wherein a width ofthe flat inner face is larger than a minimum wall thickness of theprotrusion, the width and minimum wall thickness measured in aperipheral direction.
 11. A piston for an internal combustion engine,the piston comprising: a piston body; a crown portion extending below atop surface of the piston body, the crown portion including a pistonbowl having a generally concave shape and extending within the crownportion and a wall, the wall extending peripherally around the pistonbody; a plurality of protrusions disposed in the piston bowl adjacentthe wall, each of the plurality of protrusions including a first sidesurface and a second side surface; and at least one ledge extending atleast partially through an inner face and towards an outer face of thewall, the at least one ledge extending at a depth that is less than adepth of the piston bowl.
 12. The piston of claim 11, wherein the atleast one ledge is disposed between two adjacent protrusions from theplurality of protrusions and occupies a length along an inner peripheryof the wall.
 13. The piston of claim 11, wherein the at least one ledgeis formed in segments, each segment spanning a chordal distance along aninner circumference of the wall between adjacent protrusions.
 14. Thepiston of claim 13, wherein each segment is curved to follow a contourof the wall.
 15. The piston of claim 14, wherein each segment has aperipheral width along a radial direction that is less than a thicknessof the wall in that direction.
 16. The piston of claim 11, wherein eachof the plurality of protrusions further includes a generally flat innerface that protrudes radially inwardly towards a center of the pistonbowl, the flat inner face disposed between the first side surface andthe second side surface.
 17. The piston of claim 16, wherein the flatinner face is defined between two sharp edges.
 18. The piston of claim17, wherein the two sharp edges are disposed along an interface betweenthe generally flat inner face and radially extending walls disposed oneither side of the protrusion, wherein the radially extending walls meetthe first and second side faces of a corresponding protrusion form theplurality of protrusions.
 19. The piston of claim 17, wherein the twosharp edges are disposed along interfaces between the generally flatinner face and the first and second side faces.
 20. The piston of claim16, wherein a width of the generally flat inner face is larger than aminimum wall thickness of the protrusion, the width and minimum wallthickness measured in a peripheral direction.